A laser is an optical source that emits photons in a coherent beam. Laser light is typically a single wavelength or color, and emitted in a narrow beam. Many materials have been found to have required characteristics to form the laser gain medium needed to power a laser, and these have led to innovations of many types of lasers with different characteristics suitable for different applications.
A semiconductor laser is a laser in which the active medium is a semiconductor. A common type of semiconductor laser is formed from a p-n junction, a region where p-type and n-type semiconductors meet. The semiconductor laser is powered by injecting electrical current into the gain region.
The gain region is surrounded by an optical cavity. An optical cavity is an arrangement of minors, or reflectors that form a standing wave cavity resonator for light waves. Optical cavities surround the gain region and provide feedback of the laser light.